As is known in the art, High speed FETs (Field Effect Transistors) and HEMTs (High Electron Mobility Transistors), that are used for high speed microwave and millimeter-wave systems and applications, generally require that the gate channel length be as short as possible in order to increase the operating frequency of the transistor because the short gate channel length reduces the distance that electrons transfer under the gate between the source and drain regions. In many application, a T-shaped gate electrode (sometimes referred to as a T-gate or mushroom gate) is used for these high speed transistors because they have a small stem making Schottky contact to the semiconductor, to thereby define the gate channel length, but have a much larger top metal to provide low gate resistance.
As is also know in the art, conventional electron beam or chemical photolithographic processing methods used to form sub 100 nm T-gates include the use of a sequence of photolithographic masking steps typically including coating the semiconductor structure or substrate with three different layers of photoresists. Each photoresist layer has a different sensitivity to electron beam exposure thereby enabling selective removal by the electron beam. As shown in FIGS. 1A-1C, three different electro-beam photoresist layers have been coated; each having a different sensitivity to electron beam exposure. After exposing the coated photoresist layers with an electron beam, each layer is developed using a chemical developer starting from top layer, middle layer and bottom layer in sequence as shown in FIGS. 1B-1D. It is noted that length of the top of the T-gate is defined in FIG. 1C and that the length of the stem of the T-gate and hence the gate channel length is defined in FIG. 1D. Metal is then deposited using evaporation technology resulting in the structure shown in FIG. 1E. The metal layer on top of photoresist is then lifted off and T-gate is thereby formed on top of substrate as shown in FIG. 1F. This method has several drawbacks. First, due to high aspect ratio between the cross section of T-gate top and the length of the gate making Schottky contact with semiconductor material, the mechanical stability of the T-gate is weak. This mechanically weak T-gate structure therefore has a high probability of being damaged by any subsequent processes, and thereby leads to low chip yield. A second drawback of this T-gate formation process is that the gate length may vary by subsequent processing; for example, oxygen ash process. More particularly, after development of the photoresist, any residual photoresist is removed using an oxygen plasma process. During the oxygen process, however, the oxygen process may also etch off the photoresist that is patterned by photolithography to form the T-gate before metal deposition. Without using this oxygen process, the gate metal could be deposited on top of photoresist causing poor transistor performance. A chemical recess etch process, which is needed before gate metal deposition to fabricate FETs (Field Effect Transistors), may alter the T-gate formation and lead to non-uniform gate length formation and eventually poor yield. A third drawback is that the subsequent processing after T-gate formation may later damage the exposed area between gate metal and source/drain metals. For example, a subsequent photolithographic process may be left on the semiconductor surface as residues when a conventional T-gate process is used to form T-gate. It is very difficult to clean up any of these residues around a sub-100 nm T-gate without damaging or altering the T-gate and the semiconductor surface. A fourth drawback is that electron-beam photoresist has relatively poor film adhesion to substrate and therefore the conventional processing method used to form T-gates may introduce chemicals between the photoresist and substrate, and causes the photoresist peeling off from the substrate.